What is the 866A vacuum tube?

The 866A is a rectifier vacuum tube designed for rectifier applications. Heavy duty mercury vapor. Transmitter rectifier..

What is a 866A substitute or equivalent?

Direct equivalents to the 866A include 866. These share the same pinout and operating envelope, so they can usually be swapped without circuit changes — though batch-to-batch variations between manufacturers (NOS vs current production) may shift the sonic character noticeably.

Is the 866A still in production?

Production status varies by year and manufacturer. The current Amperatubes catalog classifies the 866A as Moderate availability. NOS (new-old-stock) examples from RCA, Mullard, Tung-Sol, GE, Sylvania and Telefunken trade among collectors; modern production where it exists comes mostly from JJ Electronic, Electro-Harmonix, Tung-Sol (New Sensor), Genalex, and a handful of small-batch makers.

What does the 866A sound like?

Heavy duty mercury vapor. Transmitter rectifier.. Sonic character also depends heavily on the circuit operating point, the surrounding tubes in the stage, and the output transformer when present — use the interactive loadline calculator on this page to explore the trade-offs.

866A Rectifier Vacuum Tube

Also known as 866

Full-wave rectifier for vacuum tube power supply designs

RectifierHeater 2.5 V · 5 AUX-4RCA·GE 1934

Max DC output current: 0.5A
Peak inverse voltage: 10000V
Peak anode current: 2A
Max plate voltage: 2500V

866A

Characteristics

Datasheet values, once each. Every value cites its sheet.

HeaterDirectly heated

Specifications of 866A — Heater
Heater voltageVh	2.5 V
Heater currentIh	5 A

Rectifier limitsDatasheet maximum ratings — not to be exceeded

Specifications of 866A — Rectifier limits
Io max (DC output current)	0.5 A
Peak inverse voltage (PIV)	10000 V
Peak anode current	2 A
Tube voltage drop at Io	10 V
Max Plate V	2500 V

Mechanical

Specifications of 866A — Mechanical
Base	UX-4
Rarity	Moderate
Pinout

datasheet cited · model or editorial reading · catalogue value, no citation attached yet · nothing invented

Using this tube

Interactive loadline calculator not available for this tube yet — it needs calibrated Koren or Ayumi parameters.

Applications

Power supply rectification

Generic rectifier rectifier stage

Replace or compare

Sources

By Jean-Michel Falligan DevergneReviewed May 29, 2026Source : cap audit 2026-05-09

Manufacturer datasheets

6 manufacturer datasheets cross-checked for 866A6 verified

866A DatasheetLocal mirror · PDF

SPICE models

Revision and verification status

Source: cap audit 2026-05-09
Reviewed: 2026-05-29
Status: DataModelCalc
Model: under review

Context

The 866A is a directly-heated rectifier from RCA·GE, introduced around 1934. Commonly used in rectifier applications. Heavy duty mercury vapor. Transmitter rectifier.

History

Introduced: 1934
Manufacturer: RCA·GE
Era: Pre-War

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